There are various high molecular weight olefin polymers and copolymers which are suitable in making extrusion-cast films for commercial purposes. Such extrusion-cast films are generally made by extrusion of the molten polymer through a slot-die onto revolving chilled rolls; the general technique is often referred to as "chill roll extrusion", "cast film extrusion", or "slot-die extrusion".
There are, basically, two types of olefin polymerization techniques for preparing high molecular weight olefin polymers and copolymers. The oldest commercial technique involves high pressure, high temperature, and the use of a free radical initiator, such as a peroxide; these type polymers are generally known as low density polyethylene (LDPE) and are also known as ICI-type polyethylenes. These LDPE polymers contain branched chains of polymerized monomer units pendant from the main polymer "backbone" and generally have densities in the range of about 0.910-0.935 gms./cc.
The other commercially-used technique involves coordination catalysts of the "Ziegler" type or "Phillips" type and includes variations of the Ziegler type, such as the Natta type. These catalysts may be used at very high pressures, but are generally used at very low or intermediate pressures. The products made by these coordination catalysts are generally known as "linear" polymers because of the substantial absence of branched chains of polymerized monomer units pendant from the main polymer "backbone", and they are also generally known as high density polyethylene (HDPE). It is these "linear" polymers to which the present invention pertains. Linear polyethylene (HDPE) ordinarily has a density in the range of 0.941 to 0.965 gms./cc.
In particular, the present invention pertains to "linear" type ethylene polymers wherein ethylene has been polymerized along with minor amounts of alpha, beta-ethylenically unsaturated alkenes having from 3 to 12 carbons per alkene molecule, preferably 4 to 8. The amount of the alkene comonomer is generally sufficient to cause the density of the polymer to be substantially in the same density range as LDPE, due to the alkyl side chains on the polymer molecule, yet the polymer remains in the "linear" classification; they are conveniently referred to as "linear low density polyethylene" (LLDPE). These polymers retain much of the strength, crystallinity, and toughness normally found in HDPE homopolymers of ethylene, but the higher alkene comonomers impart high "block" characteristics to extrusion-cast films and the high "slip" (i.e. low coefficient of friction) characteristic inherently found in HDPE is diminished.
Slip and antiblock agents are often added to LDPE film compounds to insure proper film handling characteristics. "Slip", basically defined as the coefficient of sliding friction, may be adjusted by incorporating certain additives, e.g., fatty acid amides. "Blocking", the tendency of film to stick to itself, can be reduced by adding finely divided inorganic fillers such as silica. HDPE homopolymers with densities of above about 0.940 gm./cc. generally show good slip properties (that is, they slide across each other with ease), and slip agents are not normally required.
Closely related art is found, e.g., in U.S. Pat. Nos. 2,991,265; 3,324,060; 3,463,751; 3,470,122; 3,595,827; and 3,362,839.
Other art, believed to be of less relevency is found, e.g., in U.S. Pat. Nos. 3,658,980; 2,879,244; 3,070,462; 3,121,914; 3,387,073: and 3,499,950.